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Rietveld Refinement. Why do Reitveld Refinement?

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Presentation on theme: "Rietveld Refinement. Why do Reitveld Refinement?"— Presentation transcript:

1 Rietveld Refinement

2 Why do Reitveld Refinement?

3 What are the limitations of Rietveld? Phase detection limit of 1-5% Impurity atom determination New/unknown crystal structures Determination of disorder parameter Multiple parameters can have same effect diffraction patterns Equipment (powder vs. single crystal, x-rays vs. neutrons) Garbage in, garbage out! (noisy data, poor quality sample, poor sample prep)

4 Basic refinement procedure Experimental diffraction pattern Starting crystal structure (.cif, ICSD) Instrument file (.inst, LaB 6 standard) Refinement program: GSAS, FullProf Refine: Background Lattice parameters Peak intensities Peak shapes Peak positions Phase fractions Assess: Goodness of fit/R factors Impurity phases Peak/background shapes Difference pattern

5 Some common challenges/problems False minimas Refinement diverges (“blows up”) Which goodness of fit to choose? R vs. Chi sq? Preferred orientation Large amorphous background Incorrect starting crystal structure or lattice parameter Poor quality data!

6 Resources and links GSAS/EXPGUI: html html GSAS tutorials: _Crystallography/ _Crystallography/ FullProf: PowDLL (file conversion): Lots of info about fitting parameters: ements.pdf ements.pdf People: Lauryn Baranowski Lakshmi Krishna

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